Symbiosis
by tangerinee
Summary: Dean needs to kill, or the Mark of Cain will kill him. Castiel needs to steal other angels' Grace in order to survive. They find a solution that solves both of their problems. [mid Season 10 AU]


**A/N:** First of all, shout-out to my actual real life friend, tumblr user _peperhaides,_ who originally came up with this idea and let me use it. A huuuge thanks also goes to tumblr user _aquaeryn_ , who beta'd this fic and also came up with the title (because I'm uncreative af).

This story is also posted on AO3 and Wattpad (links on my profile page).

* * *

Dean groaned as he got out of the Impala to fill her up with gas.

The Mark of Cain burned like a hot iron on his forearm, the pain coming and going, only flaring up occasionally but violently. His...everything ached, and overall, he felt like shit. Maybe it hadn't been such a good idea to insist on driving… But Sam had gone to check out what seemed to be a simple salt and burn a few states over, and there was no way he was letting Cas drive his baby. He wasn't risking her for something as mundane as a grocery run.

Besides, Cas wasn't doing too well himself, with that borrowed Grace gradually burning out. He was just better at covering it up than Dean was. Sam probably hadn't even noticed, or else he might not have left the two of them alone.

"I will go inside and see if they have anything interesting," Cas stated as he got out of the car.

"Yeah, you go ahead, I'll be right there in a minute."

With a worried glance in his direction, Cas took off for the building. Once he was out of view, Dean sighed and leaned heavily against the Impala.

He had tried his best to hide it as much as possible (which he sucked at), but he didn't think he'd last that much longer, unless they did something about the Mark, and soon. Yesterday he had started coughing up blood again, and Dean wasn't too eager to find out what the next stage after that was.

Good thing Sam wasn't here right now. He knew Sam did his best, but Dean could see that his brother was desperate, out of ideas. Sam hated to be that helpless. Which is why his little brother had been so eager to go on that little hunt alone – he could actually _do_ something for once and make a visible difference.

It was a simple case; everything would – most likely – go without incident, and yet Dean would have been a burden in his current state. Plus, since he couldn't actually _kill_ a ghost, not in the way that mattered anyway, the hunt wouldn't do Dean any good anyway.

After he finished filling up his car, Dean strolled towards the building, doing his best to muffle a cough as he entered. The jingle of a bell above him announced his entrance, and Dean started looking around. When he didn't spot Cas right away, he was about to call out for his friend, when said angel was flung into the wall next to him.

"Cas...!" Dean started, but before he could say more, an invisible force grabbed hold of him and sent him flying across the room like a rag doll. A shelf on the other side of the store stopped his flight in a rather painful way. His head hit something hard, and Dean could feel blood trickle down the side of his face.

"Son of a bitch," he hissed, trying to get up and failing miserably. Dean's head hurt like hell and his vision was a little fuzzy, which was worrying. Also, the arm that bore the Mark started to feel like it was on fire again.

That's what he got for trying to _"relax a little"_ and _"do something normal for once"_ , a sour part of Dean's mind grumbled. Sam's voice and his "brotherly concerns" echoed through his head.

Bitch. He had no idea what Dean was going through. _Yeah, thanks a lot, Sam, staying behind turned out to be so relaxing_ , Dean thought bitterly. He'd show him. Dean wasn't broken; he wasn't some delicate flower that needed to be shielded from the world. Others would've cracked under the pressure a long time ago, so all things considered, Dean was handling the Mark relatively well. He'd like to see Sam try dealing with the damned thing.

 _Stop thinking about Sam and focus_ , Dean chastised himself. He could _do_ this. Blinking furiously, Dean tried to dissolve the haze in front of his eyes.

Once his vision was somewhat clear, Dean struggled to his feet. The shelf he had landed on was a broken mess, and cans of tinned food were littered around him. Kicking a few of them to the side, Dean stood up straight and took in the room.

His breath caught in his throat. There, on the other side of the room, was Cas, pinned to a wall by a slight young woman, who, by the looks of the angel blade she held to Castiel's throat, was an angel. Castiel's own blade lay discarded on the floor, just out of his immediate reach.

Muttering soft curses, Dean reached for his gun, to at least distract her long enough so Cas could break free. He cursed even more when he realized he didn't have his gun on him, or any weapon for that matter: Sam and Cas had recently deemed it best to keep weapons out of his grasp, unless they were on a hunt since they did funny things to his blood craving mind.

Being out of other options – since sneaking up on an _angel_ was bound to fail – Dean did what he does best: provoking powerful supernatural creatures.

"Oi! Feather ass!" he yelled, picking up a tin can and throwing it at her for good measure. The angel made a surprised sound when it hit her and spun her head around, hissing in his direction… which was all the distraction Cas needed. Using her temporary inattention, he stumbled free. When angel lady's attention snapped back to Castiel, Dean darted across the room, picked up Cas' blade on the way and slammed her into the wall. He held the blade to her throat in a mirror of the way she had Cas pinned seconds ago. Wasn't irony a bitch.

Dean opened his mouth, ready with a smart-ass comment, when the hand clenched around the blade started to shake. A wave of blood lust and violence crashed into his mind, infecting his thoughts and emotions. Suddenly, chopping the head off the bitch in front of him, then hacking into the remains seemed like marvelous idea. Why waste time talking to her, she was as good as dead anyway. The Mark started to glow and it's painful fire from before turned into something more...pleasant.

"Dean Winchester," the angel growled just then, snapping Dean out of his sudden, homicidal thoughts. _What the hell?_ The glow and sting of the Mark subdued as quickly as they had come as well. "You will pay next. Without you, Castiel would never have turned from Heaven," she continued, unaware of Dean's inner turmoil. With that, she threw him off of her and Dean went flying for the second time that day.

He threw up his arms on instinct, trying to balance himself and hopefully control his impact this time but ended up nicking the angel's throat with the blade as she shoved him away. Instead of blood, Dean caught sight of a faint blue glow before he crashed once again into some shelves. Various snacks rained down around him. He wanted to get up, grip the blade he dropped on landing, and get in a more defensive position. Whatever the other angel was here for, having a weapon ready seemed to be a necessity.

But a sudden coughing fit kept Dean on his knees, like a dog. Dimly he noticed that the floor beneath him was speckled with blood, but mostly he was busy staying semi-upright. Everything just _hurt_ so much. A growing ringing in his ears drowned out whatever words Cas and the other angel were exchanging. He felt like he was burning, and it was too _bright_. Even with his eyes clenched shut, the now blueish brightness didn't seem to lessen much, and it made his head feel like someone had run a sword through it...

Despite his efforts, when he became aware of his surroundings, he was lying on the floor, curled into himself and twitching slightly. The next thing he noticed was the hand gently shaking his shoulder. Flailing and only half-awake, Dean tried to get up, away from the unknown person, and grab something he could use as a weapon. But then the voice reached him and he relaxed. Cas.

"Dean?" he asked, sounding worried. How long had Cas been trying to get his attention for him to be that concerned? Did he freakin' pass out?!

"Yeah, 'm here, all good," he finally managed to get out, which was of course total bullshit. Cas' look told him that he thought so too. Sitting up and finally remembering their situation, Dean tensed.

"Where's the other ang-" Another brutal coughing fit interrupted his question, and when Dean looked at the hand he had used to cover his mouth, it was smeared with blood. The dizziness from earlier had returned as well, and suddenly just sitting there became too much. If Cas hadn't steadied him, Dean surely would've crumbled to the ground again.

Another "Dean?" reached him through the dull haze, worry mixed with panic this time. Then Cas' hand was on his forehead, glowing with a soft light and before he could take another breath, the pain and the dizziness were gone, just as abruptly as they had arrived.

Dean blinked a few times to adjust. He wouldn't say he felt good, but he didn't feel like he was gonna pass out or _die_ any second anymore either. Still most prominent was the Mark on Dean's arm, which was throbbing painfully, almost like it was angry. Dean turned to face Cas, who was crouched beside him and just stared at him with an unreadable expression. Strangely enough Cas looked...fine. No injuries from his fight with the other angel, who Dean turned to next. She was still where Dean had last seen her, only now she was lying on the ground, unmoving but still breathing.

"What...," Dean began, turning back to Cas, but then he stopped himself, as the puzzle pieces clicked together in his mind. "You took her Grace," he said instead. It wasn't a question, so Cas didn't answer and went right back to business himself. "I can temporarily dampen the symptoms of the Mark, but I can't make them disappear completely, nor can I stop them from appearing again. Come," he said, standing up and extending a hand toward Dean to help him up.

Dean thought Cas was getting ready to leave, so he followed. He frowned when he realized that Cas wasn't actually going for the door, but for the angel on the floor. They came to stand beside her limp body. Upon closer inspection Dean noticed, that she wasn't unconscious like he originally had assumed. Her eyes were open, their hateful glare directed towards Dean and Castiel, who both looked down on her. The fact that she didn't try to speak and her limbs seemed kind of stiff, made Dean suspect that Cas used his new mojo to render her completely immobile.

Before Dean could ask Cas what he planned to do, Cas turned around to face Dean and handed him his angel blade. With his head he motioned toward the angel on the ground. "Go ahead."

Dean understood what Cas was saying ( _was offering_ , a part of his mind sang), but he suddenly felt frozen in place. He would like to say it was because he was scandalized that Cas was basically asking him to execute a – for the moment – defenseless being. But what really kept him locked in place was the conflict in his head – the righteous, moral part on one side against the other part, that _craving_. It had been stirring in the back of his mind for a while now and was finally coming to life with full force, refusing to be fought down and ignored any longer.

It was screaming for blood, singing with joy for this opportunity Castiel was giving them, urging him to take the blade in his hands and kill the bitch in front of him – she had attacked them first, it was justified. Stab the blade right through the heart, or maybe the eye or slide it slowly across her throat. Maybe drag out her death real long and shred her to pieces first, coat himself in her blood. Whispered promises in his mind told him how good it would feel, how good _he_ would feel.

Effectively rendered immobile by his clashing desires, Dean started to shake when he felt Cas' hands gently pushing him down onto the dirty tiles. Kneeling next to the trapped angel, Cas tenderly adjusted Dean's wavering grip around the angel blade while whispering soothingly into his ear and telling him that it was _okay_.

Cas' reassurances, and the promises he could still hear in the back of his mind, were what broke Dean's last dam. He tore into the angel and the Mark flared brightly on his arm.

And the new part of his mind was right. It _was_ good. He _did_ feel good. For the first time in what felt like ages, Dean didn't feel like he was going crazy or dying. A feeling of peace coursed through his body, overwhelming his conscience, which was shocked by what he had just done.

Everything was clear and focused and simple as he looked up at Cas. Dean took a couple of deep breaths before speaking. "I'm better now. Thank you," he sighed with relief, a smile forming on his face and his eyes shining with new life, as the pressure of weeks left his body.

Castiel tentatively smiled back.

* * *

The interior of the Impala was eerily quiet. For once, Dean hadn't turned on the radio and was staring on the road in silence; Castiel looked out of the window, watching the landscape fly by as they left the gas station – and the empty vessel of his dead sister – behind them.

 _Another angel was dead because of him._

Castiel felt sick, and wasn't sure what to think about the earlier incident. The new Grace hadn't quite settled yet and he could feel it churning within him.

"Dean, I...," he started, but was interrupted.

"Save it, Cas. I know what you want to say. But you didn't do anything wrong." He waited if Dean would elaborate.

When he didn't, Castiel voiced the question that was bothering him. "But...do you truly believe we did the right thing?"

Dean sighed, rubbing his face with one hand, before answering. "Look, the way I see it, _she_ attacked _you_ , right? With the intention to _kill_ you, and me too, probably. So it was gonna be us or her anyway."

Dean paused for a moment before he added, "It's called self-defense, Cas. There's nothing wrong with that." But his voice sounded strangely flat at the end, like he didn't really believe what he was saying and his face looked sickly and pale against the traces of blood that remained.

After a moment of silence, Castiel opened his mouth to say something, but Dean spoke up again. "Dude, I know what you're probably thinking. You think that maybe you could've saved her somehow. But she looked pretty convinced of her cause, man. Nothing you said would've gone through to her. There's _nothing_ you could've done. It was her, or us," Dean stated again, nodding quickly, as if to convince himself as well.

Castiel considered this. Logically he knew Dean was right, but he still felt guilty about the whole thing. Part of it stemmed from his belief that his sister was right. He _was_ responsible for many of the awful things that had happened over the past few years.

But that wasn't all of it. What disturbed Castiel the most, was his own...cold behavior. Once his sister had flung Dean away, he hadn't been thinking anymore. He had seen the shimmer of Grace light and just _acted_.

"I still could have handled the situation better. I realize killing her was necessary, but that doesn't excuse the way I acted. Her Grace...there was no need...I don't know..."

"No, Cas, _do_ you know. And there was _every_ need. You've been trying to hide it, and you most likely fooled Sam – but I know you've been getting worse again. That stolen Grace you got from Crowley was burning out, wasn't it."

It wasn't a question, but Castiel still nodded, feeling bad for having been caught. He hadn't wanted to add to Dean's worries and thus had pretended to be fine.

"See? It only would've been a matter of time before you and I were equally close to biting the dust." Although Castiel didn't like it, he was forced to agree with Dean's reasoning.

Dean hesitated before he continued. "You know why you took her Grace, Cas. And we also both know why you dragged it out and let me...kill her." He said the word "kill" like he didn't think the word really described what he did and Castiel shot him a quick look that Dean tried his best to ignore.

That was another thing he couldn't comprehend. How he had urged Dean to take another ones life; how he stood by peacefully as Dean had slaughtered her. The savage way in which the hunter had torn into his sister hadn't bothered him in the slightest. Where had this coldness come from? Was it a consequence of the stolen Grace eating away at him? Or of him becoming more human in this unnatural way, maybe? Or, perhaps, it was because he actually _had_ been human for a while. Castiel didn't know the answer, but he hoped one of his theories was true. It would help to explain and justify his behavior.

"I'm not gonna start lying to you now, Cas," Dean went on, oblivious of Castiel's emotional struggle. "Even if it hadn't been her or us...I don't regret killing her."

Castiel's eyes widened. Although he had been cold before, at the gas station, he wasn't now. Right now, the prospect of his siblings being killed did disturb him. Although less then it should, frankly (he decided to ignore that for now).

Before he could reply to Dean's comment, Dean continued, having picked up on his bewilderment. "Let me explain. Yeah, I know it's selfish, but for the first time in months, I feel...fantastic. And I'd rather feel fantastic because I ganked some supernatural bitch, who already lived millennia and is whining about her current situation anyway, than feel fantastic because some poor guy happened to run into me, when I'm at my lowest."

Dean gripped the steering wheel tighter at that as Castiel mulled over his words. Despite some lingering remorse over the death of yet another sibling, he couldn't deny the truth in Dean's words. Her death had stopped Dean's suffering for the moment; her Grace would enable him to better look out for and help Dean, at least for a while.

This practical line of thinking, Castiel realized, was responsible for his inner turmoil. If he was being honest with himself, his and Dean's actions didn't bother him that much, and that's what was troubling him. Shame and disgust fought for attention in his mind, because how could he be okay with what had happened? But Dean was right, and – pretense aside – they seemed to have similar thoughts on the matter, so Castiel decided to stop deluding himself. He'd accept the way things were now.

"This isn't a permanent solution," Castiel finally said. Dean just nodded curtly. Some of the tension that had been in his body since the start of their conversation, disappeared, and Dean slumped a little in his seat. He seemed almost...disappointed. "Yeah, I know, I kn-"

"But perhaps we could make it one," he interrupted before Dean could talk himself down. "At least until we manage to actually...fix our situation." Dean's head snapped around to look at him, eyes widening in comprehension. A part of Castiel still expected his proposal to be met with contempt, but there wasn't any in Dean's eyes. Apparently he had guessed right when assuming that Dean had drawn similar conclusions out of the incident.

Dean turned to face the road again. Castiel waited to see if he would comment, but he didn't. Dean appeared to be lost in his thoughts. The silence, now without the tension from earlier, stretched on like the road in front of them.

"I don't think we should tell Sam about this," Castiel remarked after a while.

Dean snorted. "Yeah, no kidding."

"Well...what _do_ we tell him?"

His question was met with silence for the longest time. In the end, eyes not leaving the road, Dean responded with finality, "Nothing. Sammy wouldn't understand; he'd try to talk us out of it. He'll know something is up though – of course – so we can't leave it at that." Dean sat up straighter, seeming more focused now that he had a goal in mind. Adjusting his grip on the steering wheel he added, "When we get back to the bunker, grab your things. We're leaving."

* * *

Leaving the bunker, the closest thing they had to a home, hurt more than expected. But it was necessary, and not just because of Sam; staying in one place just wouldn't work for what they had planned. They kept their phones so old contacts could still reach them (after all, Dean saw no point in quitting regular hunting), even though getting new ones would've made avoiding Sam easier.

Naturally that meant Sam trying to call them 24/7, at least that's what it felt like – in the beginning.

He stopped trying after two months. Dean didn't know whether to be pissed or relieved.

Whatever he decided didn't stay on his mind for long, as the Mark pulled him deeper and deeper under its spell, Castiel – as always – following wherever Dean went.

* * *

"Well, that was easy," Dean said as he pulled his angel blade from the vessel standing before him. With the angel no longer there to hold it upright, the body crumbled to the ground, like a puppet whose strings had been cut. Turning around, Dean saw Cas taking care of another angel from the group that had ambushed them, grabbing it by the throat and swallowing down the Grace as it escaped from a small cut just above Cas' death grip. The angel groaned in pain. Looking at the display, Dean could relate to the sentiment. The angel's vessel, some Hispanic guy in his 40s, wore the most ridiculous pair of yellow pants Dean had ever seen. Ouch indeed.

They had been checking out a possible hunt in town, and had come to the industrial district that night to follow up on a lead. When they had entered the storage facility, it turned out that the whole thing had been a set-up. But it wasn't the first time something like this had happened. It appeared that Heaven didn't approve of their new lifestyle.

Joke was on Heaven though, because Cas was all juiced up and they didn't even stand the slightest chance. Dean strolled over to Yellow Pants, leaving his own victim behind and passing another dead and empty vessel on his way. It had been a small group, only four angels – no challenge really.

Blood dripped from the blade still in Dean's hand, leaving a red trail on the floor. Some of it had splattered onto his face during the fight, which made the blissful smile on his face look a little "American Psycho".

But he just felt so _good_. The painful throbbing of the Mark had long since been reduced to a pleasant, low burn, and even that would be gone soon. Not before they were done here, which Dean didn't mind at all. He found the feeling kind of soothing, to be honest. Who would've thought that things would turn out this well? Certainly not Sam, that was for sure. In the weeks before he left, Sam hadn't even trusted him with making a sandwich, out of fear of what he'd do with the knife.

In the calm right after a fight, Dean tried to avoid thinking about Sam. He did love his brother, but in these moments Dean's feelings towards him mingled more and more with a deep-rooted sense of bitterness and anger. The Mark made him feel _amazing_ (although if it was actually _that_ amazing or just the contrast to his previous miserable life he didn't know). And Sam had tried for so long to keep him from that, tried to prevent him from giving in.

Dean usually shied away from thinking further than that, because beneath the bitterness towards Sam he could feel something more sinister stir, getting stronger over the months they had been apart. He tried to squash that as much as possible – he still loved the idiot, dammit, it's just...he didn't get it! What did Sam think was gonna happen? What Dean and Cas were doing these days wasn't really all that different from what he and Sam had been doing all their lives. Except that Dean was fine for once – great actually. And all thanks to that scar on his arm, that Sam would love nothing more than to get rid off.

Right now, the Mark was glowing vividly, pulsing with the life Dean had just taken, and Yellow Pants' gaze was drawn to it, as Dean came to stand before him. He looked somewhere between wary and disgusted. Castiel shot him a knowing look and Dean pushed the thoughts of Sam as far away as possible. He then crouched down in front of the man, who had fallen to his knees after Castiel had released him. Cas hadn't pinned him down with his powers, he could still move if he wanted to, but apparently the guy had realized that fighting or running would be pointless. It wasn't like he could do much of anything anymore anyway, with his Grace gone.

"One of the angels fled the fight when she realized they were at disadvantage. I'm going after her," Castiel informed Dean before vanishing. And just like that Dean and the former angel were alone in the otherwise abandoned building.

"So...," Dean purred, "care to share your name?" When he didn't receive an immediate response, Dean pouted. "Rude." He raised his blade, but before he could do anything, the ex-angel decided to reply after all.

"You will kill me anyway, why tell you anything?" he spat out bitterly. Dean chuckled. "Well, right now the only thing I'm killing is time, but you're right, I suppose. But come on, I mean what were you expecting?" Dean wasn't anticipating an answer and Yellow Pants didn't disappoint, so he just kept talking. "You know, when word first reached us that Heaven was hunting us down, I was kinda worried. But so far, the whole lot of you has been doing a pretty piss poor job at accomplishing anything."

Dean paused to see if he'd get a response, some hopeless attempt to defend Heaven's honor or some shit, but all he got was a hateful glare. Cute. Dean's laughter echoed through the wide and open room. "I mean, you guys up there know what we're up to, right? And yet, all you've done so far is serve yourself up on silver plates."

While Dean did try to mock the guy, that was actually nothing but true. So far Heaven had only ever sent relatively small groups after them, and they didn't pose a challenge to Cas – not anymore. Most of the time, the two of them didn't even need to look for angels, they just marched right to their death all on their own. Why Heaven didn't send out larger groups was beyond Dean. Maybe they _did_ realize that they didn't stand a chance and wanted to keep their losses small, while still putting up a token fight.

Either way, instead of offering a comment, Yellow Pants turned his stare to the ground. That's when the flapping sound of wings announced the return of Castiel, who appeared right next to Dean again. Yellow Pants startled, but didn't look up otherwise. This was the look of a guy who knew he was defeated.

Dean started to rise to his feet again. "Not so nice anymore when you're on the receiving end of this popping-in crap, isn't it?" he grinned, before turning to Cas who was just putting a small vial, glowing with Grace light, in one of his coat's inner pockets.

While Grace itself was good, Dean had learned that ingesting too much at once could be dangerous for Cas. But with angels seeming to basically offer themselves up recently, it would be a shame to let all that Grace go to waste.

Since Grace needed some time, usually ranging from a couple of days to a few weeks, to settle down and properly integrate itself into what was already inside Cas, they had taken to storing the excess for that short while. Over time, Cas had been able to take in more and more Grace at once, to the point where he now actually _needed_ more, restoring Cas back to his old power, and eventually beyond that.

And, probably the biggest plus in this whole issue, it had restored his wings too. Which gave Castiel an advantage over all the other angels in existence, who were still stuck to the ground and had to try sneaking up on them the old-fashioned way. Silly angels.

"Well, you look well stocked," Dean remarked. "Lemme guess, you smote the crap out of her," he added, gesturing to Cas' clean, blood-free attire and blade.

"She was drawing attention. I'm not sure if somebody noticed and will investigate, I think it's best if you-"

"Why are doing this?!" Yellow Pants suddenly blurted out, finally staring right at Castiel. "Why are you helping this...filth to kill your own brothers and sisters to satisfy his needs?!"

Castiel looked as if he might say something, but Dean beat him to it. "Now just hold on a second. First off all, I'm not the only one who is 'satisfying his needs' here...well, on second thought, technically right now I am, since Cas' needs usually are a little less...ah, let's call it 'insistent', than mine," Dean smirked, wiggling his eyebrows.

That was something Dean hadn't thought about at first, back when they came up with this idea months ago. He had just assumed it would be an equal thing for them: he kills the angel and Cas takes its Grace, right? They were in this together after all. But he hadn't known about the potentially fatal side effects of too much Grace back then. And it turned out that Cas didn't actually need as much Grace as Dean needed kills anyway – in the beginning at least.

Castiel's Grace had usually lasted quite a while, depending on how much power he used in between fills and how well it "fit". (Apparently each angel's Grace was unique and could differ quite a lot from one another. Cas once compared it to blood types: the closer the Grace he ingested was to his original "blood type", the longer it would last.)

Dean couldn't always wait that long however. He had tried, at first, but dragging it out too long wasn't a good idea. Although he didn't get as sick as he used to (maybe the Mark was laying it on a little less thick now that it had Dean were it wanted him), he still got quite...trigger-happy. Or more like stab-happy. Agitated. Whatever, it was awkward in daily life, especially when you were trying to stay below the radar of a certain research nerd of a brother.

By now of course, Cas had become more dependent on Grace as well, to be able to sustain the level of power he had reached. Dean sometimes teasingly called him "Grace Junkie".

At first, Cas had needed to store the Grace he took most of the time to keep up with the Mark's demands, since he couldn't take in all that power at once. Now however things had calmed down significantly, with Cas' and Dean's rhythm almost in sync; most of the time Castiel just occasionally stored Grace as a "snack" for later.

In the beginning, things had sometimes been a little awkward. Now, Dean had no qualms about going "hunting" whenever the Mark demanded it, no matter if Cas needed a fix as well or not.

As if reading his thoughts, the Mark chose that moment to start pulsing softly again, a gentle reminder that there was still work to do. Although it had had its fill, it still wasn't going to let Yellow Pants live apparently. _Greedy little bitch_ , Dean thought to himself, a predatory grin settling on his face.

"Hey Cas?" Dean began, spinning his blade. "If you wanna say something, do it now, 'cause I got the feeling this guy ain't got that long to live." The drawl in his voice made him sound drunk, like the addict he probably was. He shook the arm bearing the Mark to get the point across to his partner.

Cas fixed his gaze on the red glow of the Mark for a moment, before looking back to Yellow Pants. "I'm helping a righteous man – who is bearing a great burden – to kill my own siblings because I am done with Heaven. This 'filth', as you call him, is worth more than any of you," he stated, calm and quiet, and still Yellow Pants flinched. There was a certain resonance to Castiel's voice these days that spoke of a power way beyond that of your usual run-of-the-mill angel. Dean found it kind of hot, to be honest. Though that may just be the Mark being drawn to the power. He didn't really care any longer.

"Over the years, you, my dear 'siblings' have tried to kill me multiple times, while I tried to do what was right. Multiple times you sought to harm humanity, the very beings Father told us to love and protect, and I always stood against it." There was a sneer on Castiel's face when he continued. "Things may be peaceful now, but how long before a new tyrant takes charge of Heaven and decides to take the fight to earth? I won't stand by to see that happen. I am done with 'my siblings'."

With those last words, Cas stepped back. That was all the invitation Dean needed. Before Yellow Pants had any chance to react to Cas' little speech, Dean plunged his angel blade into the guy's heart.

A wave of warmth, power and satisfaction rolled through his body, as sweet as a lovers' embrace, and Dean sighed in content. The new rush of energy made him feel hyper and delirious, and Dean couldn't understand why he ever resisted this. He turned to Cas and clasped him on the shoulder, just shy of giggling.

"So. I saw a nice diner when we arrived in town; you think it's still open?"

* * *

Castiel stood by the window, watching the storm rage outside. The occasional lightning flash illuminated the motel's almost empty parking lot. Although it was daytime, the storm made it look dark outside, and the horrible weather conditions had forced them to find a place to stay, since according to Dean, _"driving through this crap just isn't any fun"._ Besides, they weren't in a hurry anyway. Behind him in the room, he could feel the nervous energy the hunter was giving off.

Dean was getting restless again. That in by itself that was nothing unusual; it simply indicated that soon he would have to kill again, to satisfy the Mark. What worried Castiel, was that those phases of satisfaction seemed to be getting shorter, and he had no idea why. There didn't seem to be anything wrong with Dean; he had subjected him to a thorough Grace examination to make sure – unbeknownst to the hunter, who would, without a doubt, have claimed that he was "fine".

They haven't talked about the issue yet, but Castiel was sure Dean had noticed it as well. At least if his discomfort, and his recent attempts to hide his needs and delay the killings were anything to go by. Although Dean didn't have any problems with the killing itself, not anymore, Castiel knew that he didn't like to be constantly forced. While Dean had accepted the Mark, this permanent pressure made him want to resist its urges just out of spite – but also for more practical reasons: the more killings, the more difficult it would be to hide them, and drawing any attention to them was the last thing they needed, especially now that Sam had recently figured out just what they were up to.

He had caught up to them fairly quickly after that, a couple of weeks ago now. The ensuing confrontation hadn't been pretty for either brother. Neither had been ready to accept the others point of view. When the conflict had threatened to escalate, with Sam trying to take Dean with him – to _"make him see reason"_ – by force, Castiel had intervened and simply flown himself and Dean away. Sam probably hadn't expected that; as far as he was concerned, Castiel was still just another normal angel, even though he was so much more than that these days.

Although they haven't heard from Sam since then, they were sure that he was trying his best to pick up their trail. He and Dean haven't talked about what they would do if Sam did catch up to them. For Sam's sake, Castiel simply hoped that he wouldn't cross paths with them again.

But his musings about things that couldn't be changed wouldn't do anybody any good. Castiel abandoned his place at the window and turned to face Dean, who paced around the motel room like a caged animal.

"Dean," he started. He figured that not-talking about the problem at hand wouldn't help either. Dean stopped his pacing next to the table and turned to look at him. Then he sighed and let himself sink into one of the room's chairs, massaging his temples.

"Sorry man, I know, it's just...I'm feeling..."

"You need to kill," Castiel stated for him.

Dean looked up at him with a strained smile. "That obvious, huh. But yeah, that's it. It's just...I don't get it, Cas!" Dean exclaimed, furrowing his brows. "I ganked that last angel, like, what? Two or three days ago? I should be all well and swell and sated right now, dammit!"

He slammed his fist onto the table, then let his head sink into his hands. Castiel sat down on the other chair, opposite of Dean. Before he could say something reassuring, Dean continued. "Hate to say it, man, but maybe Sam had a point. Maybe this isn't how we should-"

"No!" Castiel cut in sharply. Dean looked up, surprised at the sudden outburst. "No, Dean," Castiel repeated, to really drive it home." Your brother doesn't understand our situation. We _need_ this. You especially were in a horrible state, when we were still traveling with Sam. We are doing the right thing, Dean, you showed me that."

It was important that Dean absolutely understood this. He couldn't start to doubt, not now, not after everything they went through and have done together. They were in far too deep to ever go back to how things used to be. And honestly, Castiel wouldn't want it any other way. These past months with Dean, he had felt a freedom he hadn't ever known before.

The silence between them was only filled by the sound of rain outside. After Dean deflated and nodded his assent, Castiel continued, "There is nothing we can do about the Mark itself, but we should find out what changed, why you don't seem to get the same satisfaction out of killing anymore." Dean ran a hand trough his hair. "Yeah, you're probably right," he sighed. "So. What _exactly_ do we do now?"

"I don't know. This is as new to me to me as to you. It's not like we have any precedents for this kind of situation. We can only try to analyze it, to spot the problem."

"Right..." Dean muttered, biting his lips and absently rubbing the forearm that bore the Mark. Castiel followed the movement with his eyes. He no longer viewed the Mark with disgust, like he used to not too long ago.

Instead, he found himself fascinated by it and the glow it emitted when Dean went in for the kill, mirrored in a glint of the hunter's eyes and a smile on his face: sometimes a vicious grin, sometimes a serene upturn of his lips. There was a certain elegance in the way Dean killed; it was mesmerizing to watch.

His change of heart regarding the Mark of Cain consisted of multiple factors. For one, it was responsible for their new life, which he so loved. Dean too seemed to be happier and more carefree than Castiel had seen him in years. And then there was also the fact that the Mark made Dean better. Stronger. A normal human couldn't even hope to win against angelic opponents, whether weakened by the Fall or not.

Dean however relished in the fights, his strength and reflexes greatly improved. He was a predator, and his prey didn't stand a chance. It was a relief for Castiel to not have to worry about his friend during their hunts. Also, with the ever-growing amount of power he had at his disposal, he wouldn't want to deny Dean this.

"Right," Dean said again, seeming to snap out of a daze and pulling Castiel out of his own thoughts as well. "So, this has been going on for quite a while, although I didn't notice it right away because it progressed pretty slowly, I guess? But...I don't get it. I mean, at first I thought it was because I wasn't a demon anymore – that I somehow can't shut the Mark up as a human. But it worked before, before Metatron killed me. So what's different now?" Dean finished, looking puzzled.

The lightning outside chose that moment to strike something nearby. The accompanying thunder was deafening, and Dean startled badly, instinctively drawing his angel blade as he cursed. The rumbling outside went on for a moment, the lights in their room flickering, before things calmed down again.

"Geez," Dean breathed. "Sorry. I'm kind of jumpy," he offered with an embarrassed smile. But he made no move to put the angel blade away again, and began toying around with it, twirling it in his hands.

The way the blade reflected the light of the room's lamps caught Castiel's attention and he found himself hypnotized by the sight of the blade in Dean's hand, spinning round and round and- "Dean," Castiel spoke up, tilting his head in consideration. When he had his partner's attention, the angel blade came to halt, and he voiced the question that had just occurred to him.

"How did you kill? Before?" Dean looked confused, so Castiel elaborated, "When you first got the Mark, how did you kill? What weapon did you use?" Dean's eyes widened as he caught on to what Castiel was asking.

"You think it's that? The First Blade?...I mean, I didn't use it always, heck, it took us ages to find the damn thing in the first place, but...often enough, I guess. And this...need only started to become a noticeable problem after I killed Abaddon anyway, so I had the thing at that point. And when I was a demon, I barely used anything but the Blade." Dean looked down at the angel blade in his hands, seeming to consider this new theory, before he looked up at Castiel again. "You seriously think that could be it?"

Castiel shrugged. "It's as good a guess as any. Since the Mark and the First Blade belong together, and the Blade can't function without the Mark, it would only be logical that there could also be negative side effects vice versa."

Castiel smiled as Dean beamed at him, thrilled at the prospect of solving this problem and going back to a more reasonable killing pace. "So it's settled? We actually have a plan?" Dean didn't wait for a reply and threw his head back, laughing, before he continued.

"Cas, my genius friend, we are going to look for the First Blade. And I know just the place to start." Dean grinned, a wicked light in his eyes.

* * *

Crowley was sitting on his throne, trying to not fall asleep as his advisers...advised him. Or something. He had stopped paying attention a couple of minutes after the meeting started, settling for just signing whatever papers they handed to him on occasion.

Crowley realized that these meetings were actually important to make Hell run smoothly – he was the one who established this whole bureaucracy after all – but it got _so_ mind-numbing over time. The discussions seemed to cover the same topics every damn week and did nothing but run in circles. How was that even possible? Why didn't anything exciting ever happen?

He was the King of Hell. He should not have to put up with this garbage.

Also, the noise was grating on his nerves. How many people did they currently have in the palace's torture chambers anyway? He should tell his men down there to ease up on the more physical aspects of information gathering during his meetings; they were hard enough to tolerate without the additional background noi-

"Um...sir?"

The timid inquiry jolted Crowley out of his inner whining. To cover up his own inattentiveness, Crowley shot the unimpressive, mindless goon standing before him an extra annoyed look. After the demanding rise of his eyebrow didn't get him a response, the King of Hell let out an impatient sigh.

"What?"

Mindless Goon, (whose name might be...Jeffrey? It was hard to keep track of these morons) flinched at the sharpness of the response, but it's not like Crowley cared about that. What he did care about, now that he was paying attention again, was how eerily quiet the room had gotten. Gone were the talks of before – the monotone babbling of his older consultants, as well as the more lively calls of the newcomers.

Instead, there was a nervous energy in the room, a certain tension that was underlined by the guards fumbling with their weapons. They must have drawn them at some point, while Crowley's mind had been elsewhere. Huh. That didn't look good. Although Crowley wasn't really in a position to complain, he _had_ wished for some sort of distraction from this appointment after all. Maybe he should have been more careful with his wishes.

A scream from outside his throne room made Crowley notice something else. What he had been whining about earlier wasn't the usual torture noise, which normally didn't bother him that much anyway. No, the screams he was hearing now had a different quality to them and were also getting...louder? No, wait, they were getting closer. Oh wonderful. This whole situation just kept getting worse.

"What's going on?" he asked again, more alert himself now. Jeffrey started stammering out a response. "Sir, um, we are...well, there seems...um...seems to be an intrusion, or..."

"Less talking, more facts, Jeffrey!" Crowley snapped, fed up with the idiots he was surrounded with. "Who is it, and how did they get in here in the first place. This place is supposed to be warded against, well, everything. Everything that's not working for me, that is."

Also, what kind of moron would even think about breaking into the Kings of Hell's earthly headquarters? "It's, um, Josh, not Jeffrey, sir, and that is the problem, we, uh, don't know who is responsible for this and how they managed to breach our wards." Josh looked like he'd rather be somewhere else.

"Well, why are you just standing here then? Get the hell out of here and find out!" His exclamation was followed by awkward silence. Josh seemed to shrink into himself even more.

"That's another problem, sir, we...um...seem to be unable to leave the room."

Before Crowley could snap just what the hell _that_ was supposed to mean, Guthrie, one of his older and more experienced advisers, cut in. "Some force prevents us from leaving. The door won't open, although it's technically unlocked, and our ability to teleport out of the room is blocked as well."

While Crowley tried to whisk himself away (and bugger all, Guthrie was right, it didn't work!), said adviser continued his explanation. " _In theory_ , the only beings who _maybe_ could circumvent our security measures like that, would be angels; I believe we possibly didn't renew the anti-angel sigils in a while. We saw no need, since it should be entirely impossible for them to get past even our most basic security since their Fall from Heaven. They wouldn't even be able to get into the building...!"

Guthrie went on a for while, about how even with the sigils weakened, it still shouldn't be possible for an angel invade the palace like that, but Crowley wasn't listening anymore. His mind was still stuck on the "angel" part. He was about to ask for some clarification, when the screaming on the other side of the door seemed to reach it's climax, only to die instantly afterwards.

Bollocks.

There was only one angel he could think of, that would have any reason to talk to him – although Crowley couldn't think of a _specific_ reason right now. It wasn't like he had tried to kill him or the Plaid Brigade recently. (Although speaking of which, there had been rumors recently that there was trouble in paradise. What kind of drama did Moose and Squirrel have this time?) In fact, Crowley had done _nothing_ lately that would justify anything but a social visit.

And yet, and by the sounds of it, his uninvited visitor didn't seem to be here for tea and scones. When the door burst open however, he didn't see Castiel, as he had expected, but Dean Winchester, clutching an angel blade that was dripping with just as much blood as the man himself. Wearing a manic grin, he waltzed in as if he owned the place.

The demons who moved to attack, suddenly froze mid-motion, just as Crowley heard the soft flutter of wings behind him and felt the cool edge of a blade being pressed against his neck.

"I would suggest you stay where you are," came the dry command from behind his back, and yes, that was Castiel. He would have been surprised if Squirrel came alone.

"Hiya Crowley," Dean called in a warm and pleasant voice, that didn't fit his threatening smile at all. With a couple of confident strides, Dean crossed the room and came to stand in front of the throne. He didn't seem to take any notice of Crowley's advisers, who were still frozen like someone had pressed "pause" on a DVD.

"Squirrel," Crowley replied, obvious fake-enthusiasm in his tone. "Long time no see. Looks like you found an...outlet...for your...issues."

Dean just continued to grin like the madman he probably was.

Looking at the hunter though...Crowley saw that he was soaked with blood which was probably not his own. (Indeed he couldn't find a single scratch on the man – were his demons that incompetent or was that the angel's doing?) There was a fire in his eyes that mirrored the energy he was radiating: a thirst for violence and blood.

Dean's entire presence, though self-assured, had something not quite sane to it, and it sent chills down Crowley's spine, as much as he hated to admit that. And of course there was that bloody Mark on his arm, which glowed like some joyful Christmas decoration right now. And yet, Dean looked nothing short of delighted to be here.

Scratch the "probably". That guy was definitely off the rails.

"And Castiel!" Crowley continued, trying to turn his head to look behind himself, but stopping when the sharp pressure on his neck increased. "All powered up again, I see. And how is that flying thing working, I thought you and all your feathered friends had your wings clipped."

Castiel remained silent. Instead Dean stepped forward, until he was practically looming over Crowley, who didn't dare to move a muscle. "We came here for a reason," Dean started. _Social visit?_ Crowley's mind supplied hopefully.

"See, the thing is, you have something that belongs to me," Dean continued, as he slowly leaned down to stare Crowley directly in the eyes, his blade hovering way too close for Crowley's comfort. The pressure on his neck increased, until he could feel skin break and blood trickling down, ruining his suit. Not that it would matter if he made a wrong move now.

"I want it back," Dean hissed, the pretend warmth in his voice from before replaced with venom. "And you're going to tell us exactly where it is."

 _Bloody hell, please don't be talking about what I think you are,_ Crowley thought, closing his mental eyes in resignation. When did things ever go his way? And where was Moose when you needed him? He was supposed to be the voice of reason in their ridiculous little group.

Suddenly Castiel leaned down, his lips lingering at Crowley's ear, growling in a tone that offered no room for disobedience.

"Where...is...the First Blade."

Well damn.

* * *

 **A/N:** This is my first fanfiction, so feedback would be greatly appreciated :)

(I'd also like to add that this story is _complete_ , since the only comments I'm getting are mostly demands for more.)

* * *

 _Hey, you just read this,_  
 _and this is crazy_  
 _But I'm on tumblr,_  
 _so follow me maybe?_

(link on my profile page)


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